Start quiz button is not implemented in the hook_field_extra_fields

The "Start quiz" button that gets attached to the quiz node in hook_view should be implemented in hook_field_extra_fields so that Drupal core and other modules know about it.

I found this problem when I was using this module with Display Suite (http://drupal.org/project/ds) and the "Start quiz" button wouldn't show up in the Manage Display tab in the back-end and wouldn't be rendered in the front-end.

Here is my suggestion to fix the problem:

/**
 * Implements hook_field_extra_fields().
 */
function quiz_field_extra_fields() {
  $extra = array();

  $extra['node']['quiz'] = array(
    'display' => array(
      'take' => array(
        'label' => t('Take quiz button'),
        'description' => t('The take button.'),
        'weight' => 10,
      ),
    ),
  );

  return $extra;
}